Device with rolling diaphragm seal separating gas and liquid



United States Patent [72] Inventor Herman Henricus Maria Van der Aa Emmasingel, Eindhoven, Netherlands [21 1 Appl. No. 621,847 [22] Filed March 9, 1967 [45] Patented Dec. 15, 1970 [73] Assignee U.S. Philips Corporation New York, N.Y. a corporation of Delaware. by mesne assignments [32] Priority .March 22, 1966 [3 3 Netherlands [54] DEVICE WITH ROLLING DIAPHRAGM SEAL SEPARATING GAS AND LIQUID 5 Claims, 8 Drawing Figs.

[52] us. Cl. 92/83, 92/99, 91/416, 91/433 [51] Int. Cl. F16j 3/00, P15b 15/17 [50] Field ofSearch 74/182,

(inquired); 277/(Inquired); 92/82, (Cursory), 83, 84, 98, (Cursory), (inquired); 123/(lnquired);

[56] References Cited UNITED STATES PATENTS 3,272,089 9/1966 Rietdijk 3,277,795 10/1966 Rietdijk 3,302,392 2/1967 Fukker et al.. 3,339,464 9/ 1967 Rietdijk Primary Examiner-Paul E. Maslousky Attorney- Frank R. Trifari sive pressure differential across the diaphragm.

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PATENTED [151:1 519m SHEEI 1 OF. 5

INVENTOR.

HERMAN H. M. VAN DER AA AGEN PATENIEU 0111 519111 SHEEI 50F 5' I/ III [III 'I/II III/[III] 11 III/II IIIllIIIIIIIIII/I I INVENTOR. I HERMAN 11.1mm DER AA AGENT DEVICE WITH ROLLING DIAPHRAGM SEAL SEPARATING GAS AND LIQUID Devices of the above type are known in hot-gas reciprocating engines, compressors, and the like. These machines include a rolling diaphragm seal between the piston and the cylinder in order to prevent, eithergaseous medium leaks away from the working space, or contaminations such as lubricant from entering the working space.

" In these engines the rolling diaphragm bounds a liquid space with its side remote'from the working space. Liquid can be I supplied to this space by pumping means, a control device which is capable of conducting liquid away from this space such that the pressure differential across a rolling diaphragm (which faces the space with its concave side) does not fall below a given minimum value, and that said pressure dif ferential (across a rolling diaphragm which faces the gas space with its convex side) does not exceed a'given maximum value.

In these known devices it may occur that by some cause or other the pumping means supply an insufficient amount of liquid. As a result of leakage the quantity of liquid in the liquid space will decrease. This means that in the case of a concave diaphragm the pressure differential across the rolling diaphragm will increase and will become so large in the long run, that the rolling diaphragm is deterioratedor destroyed. In the case of a convex diaphragm, on the contrary, the pressure differential will decrease as a result of which at a given instant the rolling diaphragm is no longer stretched'which results in the formation of pleats so that the rolling diaphragm will be deteriorated or destroyed.

In the known devices the pumping means are usually coupled directly to the shaft of these devices; So when the devices are not operating, no liquid will be supplied. During the inoperable period the liquid will leak away from the liquid space. When the device is started again no liquid or an insufficient amount of liquid will be present initially for supporting the rolling diaphragm. This means that when the desired operating pressure occurs immediately in the working space, the pressure differential across the rolling diaphragm will have an unacceptable value. For that reason it has so far been impossible to start the device directly with the desired operating pressure.

It is the object of the invention to provide a device having a rolling diaphragm seal of the above described type which can be started. immediately at the desired operating pressure without the rolling diaphragm being loaded to an unacceptable extent. v

The device accordingto the invention is characterized in that the said liquid space communicates through a duct with a space containing a gas and/or a liquid, means being provided for bringing medium from the said space into the liquid space when the pressure differential across the rolling diaphragm exceeds a maximum value which may be adjustable or falls below a minimum value which may be adjustable.

. If in the device according to the invention the pressure differential across the rolling diaphragm becomes too large or too small, additional medium is applied to the liquid space below the rolling diaphragm so that a good support of the rolling diaphragm is ensured at all times. As a result of this it has become possible to start said device immediately with the desired operating pressure, while in addition a safeguarding has been obtained for disturbances in the supply of liquid to the liquid space occurring during operation, so that a shortage of liquid in that space would occur and the pressure differential across the rolling diaphragm would become too large or too small. The means for supplying additional medium to the liquid space may be constituted by a pump.

In a favourable embodiment of the deviceaccording to the invention, the pressure of the medium in the said space is always higher than the pressure in the liquid space, a controllable valve being arranged in the communication duct which valve is opened when the pressure differential across the rolling diaphragm exceed s a maximum value which may be adjustable or falls below a minimum value which may be adjustable. In this case, the valve may be a spring-loaded check valve or a valve controlled by the pressure differential across the rolling diaphragm. As a result of the pressure differential, the medium automatically flows to the liquid space.

In a further embodiment of the device according to the invention,- the liquid space is in open communication with the said space through the communication duct, the space containing a liquid under a pressure which is always substantially equal to the pressure in the gas space reducedand increased, respectively, by the desired differential pressure across the rolling diaphragm.

In another favourable embodiment of the device according to the invention, in which the rolling diaphragm faces the gas space with its concave side, the said communication duct which comprises a value and adjoins the liquid space with its other side directly adjoins the gas space sealed by the rolling diaphragm. In this device, when the pressure differential across the rolling diaphragm exceeds a maximum value, that is to say, if insufficient liquid is present in the liquid space, as gas support is temporarily used instead of a liquid support.

A further favourable embodiment of the device according to the invention in which the rolling diaphragm faces the gas space with its concave side, is characterized in that the communication duct comprising a valve and adjoining the liquid space adjoins with its other side a further space which is entirely filled with liquid, which further space is separated from the gas space sealed by the rolling diaphragm by a freely movable wall or part of a wall. Consequently, the liquid in the further space will be under the same pressure as that which prevails in the sealed gas space. If the pressure differential across the rolling diaphragm exceeds the maximum value, the valve in the communication duct will be opened, so that liquid is forced out of the further space into the liquid space. As a result of this it is achieved again that the pressure differential across the rolling diaphragm always remains within the desired limits.

In a further favourable embodiment of the device according to the invention, in which the rolling diaphragm faces the gas space with its convex side, the communication duct, which comprises a valve and adjoins the liquid space, adjoins with its other side a container which contains a gas and/or liquid under a pressure which is at least equal to the pressure in the sealed gas space increased by the minimum. permissible differential pressure across the rolling diaphragm. When the rolling diaphragm faces the gas space with ,its convex side, the pressure in the liquid space will always have'to be higher by a given amount than the pressure in the sealed gas space. This means that if medium is to be supplied to said space it will be necessary that said medium can be suppliedunder a pressure which is higher than the desired pressure in that space. In order to achieve this, according to the invention, a separate container is provided in which the desired high pressure prevails.

In order to ensure that substantially immediately after opening the valve in the said communication duct the pressure in the liquid space assumes the desired value, according to a further embodiment of the device according to the invention, the communication duct is constructed so as to have a low resistance to flow.

' According to a further favourable embodiment of the device according to the invention, in which the communication duct adjoining the liquid space adjoins with its other side a gas-filled space, the said control device is also incorporated in the communication duct. This has the advantage that the gas which is used as a temporary supporting medium will first be conducted away again from the liquid space after the pumping means are actuated.

A further embodiment of the device according to the invention is characterized in that while the communication duct adjoining the liquid space adjoins with its other side a gas-filled space, at least when the valve is in its open position, substantially no pressure variations occur in the gas space sealed by the rolling diaphragm. Because gas has a certain compressibility, also the gas in the liquid space will be compressed alternately more or less, by pressure variations if these occur in the sealed space so that volume variations occur, as a result of which the rolling diaphragm will be expanded more or less which has adverse influences on the life of said rolling diaphragm.

A further embodiment of the device according to the invention, which comprises at least one cylinder and a pistonlike body reciprocating therein and varying with its one side the volume of a working space and bounding with its other side a buffer space having such a volume that the volume variations caused by the movement of the piston produce substantially no pressure variations in that buffer space, the pistonlike body being provided with a piston rod which is passed through the wall of the buffer space, a seal being arranged both between the pistonlike body and the cylinder and between the piston rod and the wall of the buffer space, is characterized in that only the seal between the piston rod and the wall of the bufi'er space is constructed as a rolling diaphragm, the other side of the communication duct which adjoins the liquid space below the rolling diaphragm and comprises a valve opening into the buffer space. This device may be immediately started at all times with the desired working pressure in the working space and in the buffer space also if an insufficient quantity of liquid is present in the liquid space for supporting the rolling diaphragm. In that case the valve in the communication duct will be opened and the rolling diaphragm be supported temporarily by gas which'is under the pressure which prevails in the buffer chamber. After some time the pumping means will have supplied a sufficient quantity of liquid, after which the rolling diaphragm is supported by liquid.

A further favourable embodiment of the device according to the invention is constructed as a hot-gas reciprocating engine of the displacer type and comprises at least one cylinder having a piston and a displacer reciprocating therein for varying the volumes of a compression space and an expansion space having different average temperatures; The spaces communicate with one another with a generator arranged in said communication so that a working medium can flow there and back. The piston bounds with its side remote from the working space a buffer space having such a volume that the volume variations produced by the movement of the piston substantially cause no pressure variations; the piston comprises a piston rod which is passed through the wall of the buffer space and the displacer provided with a displacer rod which is passed through the piston and the piston rod, with seals provided between the piston and the cylinder, between the piston rod and the wall of the buffer space, and between the piston and the displacer rod. This device is characterized in that the seal between the piston rod and the wall of the butter space is constituted by a rolling diaphragm, the seal between the piston rod and the displacer rod being constituted by a nonhermetical seal and a rolling diaphragm, the space between these two seals being in open communication with the bufier space and the two liquid spaces below the rolling diaphragms being in open communication with one another. The said communication duct comprises a valve adjoining at least one of the said liquid spaces and communicating said spaces with the buffer space. In this manner an extremely satisfactory seal of a hotgas reciprocating engine of the displacer type is obtained which can be started immediately with the desired operating pressure without objection for the rolling diaphragms, said rolling diaphragms being temporarily supported by gas from thebuffer space if an insufficient quantity of liquid is available below the rolling diaphragms. Since substantially no pressure variations occur in the buffer space, this gas support has no adverse influence for the rolling diaphragms.

After the above description it will be clear that the invention provides an extremely simple measure for starting devices which comprise rolling diaphragm seals supported by liquid immediately at the desired operating pressures, while this measure during operation serves as a safety when interferences occur in the liquid supply to the liquid space below the rolling diaphragm so that said rolling diaphragms are not exposed to too largepressure differentials.

In order that the invention may readily be carried into effect, a few examples thereof will now be described in greater detail, by way of example, with reference to the accompanying drawings which are shown diagrammatically and not to scale.

FIGS. 1, 2 and 3 diagrammatically show cross-sectional views of three embodiments of devices which comprise rolling diaphragm seals supported by liquid, the rolling diaphragms facing the sealed gas space with their concave sides.

FIGS. 4 and 5 diagrammatically show cross-sectional views of two embodiments of devices which comprise rolling diaphragm seals supported by liquid, the rolling diaphragms facing the gas space with their convex sides.

FIG. 6 diagrammatically shows a device comprising a rolling diaphragm seal, in which, when the pressure differential across the rolling diaphragm falls below a given value, liquid is supplied below the rolling diaphragm by means of an additional pump.

FIGS. 7 and 8 show two embodiments of hot-gas reciprocating engines of the displacer type which are provided with a buffer space below the piston and in which liquid-supporting rolling diaphragm seals are used.

Referring now to FIG. 1, reference numeral 1 denotes a cylinder in which a piston 2 reciprocates which is connected, through a piston rod 3, to a gear not shown. When the piston 2 moves, its upper side varies the volume of a working space 4 containing gas. The seal between the piston 2 and the cylinder 1 is formed by rolling diaphragm 5 which, at least during normal operation, is supported by liquid contained in a liquid space 6 below the rolling diaphragm. The liquid space 6 is bounded on its upper side by the rolling diaphragm and on its lower side by the two annular surfaces 7 and 8 of equal area so that, when the piston moves, the volume of the space 6 will remain constant. The piston 2 further comprises an oil pump ring 9 which pumps lubricating oil which is on the wall of the cylinder to the space 6. To maintain a constant pressure differential across the rolling diaphragm 5, a control device 10 is provided. This device consists of 'a housing 11, defining therein a control chamber which is divided into two parts by a diaphragm 12. The part 13 communicates with the liquid space 6 through a duct 14, while the part 15 communicates with the gas space 4 through a duct 16. A liquid outlet pipe 17 further communicates with the part 13 which duct can be closed by a valve 18 which is connected to the diaphragm 12. The diaphragm 12 furthermore comprises a sliding member 19 which is movable in the duct 16 and can close or open a duct 20 in accordance with the position of the diaphragm 12.

The operation of this device is as follows. During normal operation the oil pump ring 9 supplies oil to the liquid space 6. As a result of this the oil volume in the space 6 increases which would have for its result that the pressure differential across the rolling diaphragm 5 would decrease. The diaphragm 12 in the control device 10 is under the influence of a compression spring 22 which is adjusted so that the valve 18 opens the liquid outlet 17 when the pressure differential between the gas in the space 4 and the liquid in the space 6 has reached a given value, for example, 5 atmospheres. The oil then still applied by the pump ring 9 disappears immediately again through the oil outlet duct 17 out of the space 6. This so-called flushing of the oil in the space 6 has the advantage. that gas, if any, diffused,

through the rolling diaphragm, disappears out of the space 6 with the dissipated oil, so that the danger does not exist that at a given instant the diffused oil exceeds the saturation concentration so that vapour bubbles would be formed which would make the support of the rolling diaphragm' 5 somewhat resilient. So in this manner the situation is prevented where the pressure differential across the rolling diaphragm 5,

becomes smaller than a minimum adjusted value.

When the device is stationary, the liquid will graduallyleak away out of the space 6. If afterwards it is desirable to start the device again immediately at the desired operating pressure in the space 4, there will be no liquid available for supporting the rolling diaphragm 5 so that a very large pressure differential across the rolling diaphragm will occur. This is prevented -in the device shown in FIG. 1 in that, when the pressure dif;

ferential across the diaphragm 12 in the device exceeds a given value, this diaphragm is forced down through such a distance that the slide 19 opens the duct so that gas from the space 4 enters the spaces 6 and 13 through the ducts 16 and 20.80 inthat case, the rolling diaphragm 5 is temporarily supported by gas. This has no adverse influence for the rolling diaphragm. if only no or substantially no pressure variations occur in the space 4. The diaphragm will be supported and stretched in the correct manner such that the device can start at the desired operating pressure in the space 4. It is desirable that the ducts 16, 20 and 14 in this device should have a small resistance to flow so that the gas from the space 4 may enter the space 6 without delay. The duct 20 opens directly near the outlet 17, so that the gas from the space 4 which has reached the space below the rolling diaphragm can disappear out of the liquid space as rapidly as possible again when liquid is supplied by the pump ring 9.

FIG. 2 shows a device :which in outline entirely corresponds to the device shownin FIG. 1. The liquid space 6, however, in this device does not communicate through a communication duct to the working space 4, but with a liquid container 26. This container 26 is sealed-at its top by afreely movable pistonlike member 27 which with its otherside bounds aspace 28 which is in open communication with'the gas space 4. This means that the liquid in the space 26 will always be under the same pressure as that which prevails in thespace 4. When, for example, on starting no or an insufficientarnount of liquid is present in thespace 6, the diaphragm 12 of the control device 10 will be forced downwards to such an extent that the valve 29 is opened so that liquid from the space 26 is forced to the space 6. it is required in this case that the duct 25 again has a low resistance to flow so that only a small delay exists between the instant at which the valve 29 is'opened and the instant at Y which liquid from the space 26 reaches the space 6. In this device the rolling diaphragm 5 consequently is always supported by liquid which renders this device suitable as such for varying pressures in the gas space 4. FIG. 3 shows a device which again corresponds to the devices shown in the preceding FIGS. In this device the control device 10 is large so that a considerable quantity of liquid is present in the space 13. The space 13 is in open communication with jthe liquid space 6 through a communication duct 14, The space 13 is separated from the space 15 by a freely movable'pistonlike member 30, which latter space is in open communication with the gas space4. i T m- A spring 31 having a flat characteristic'eng ages the piston member 30. As a result of this the pressure and the liquid will always be lower by a given amount than the gas pressure in the spaces 15 and 4, respectively. Guides 32 are connected to the pistonlike member 30 for guiding a plate 33 to which the valve stem of a valve 34 is connected.

The operation of this device is as follows: Liquid is applied to the space 6 by the pumping ring 9. When sufficient liquid is in the spaces .6 and 13 that the liquid pressure is equal to the gas pressure in the spaces 15 and 4 decreased by a pressure which corresponds to the force exerted by thespring 31 on the piston 30, the valve 34, if more liquid-is supplied, is further opened so that the liquidis dissipated from the space 13.

high, that is to say when the pressure difi'erential across the rolling diaphragm exceeds a given value, the valve 18 will be lifted, so that oil can be conductedaway from the space 6 through the outlet 17. The liquid space 6 communicates, through a duct 35 having a low resistance to flow, with a container 36 which contains gas or liquid under a pressure which is at least equal to the pressure in the gas space 4 increased by the desired pressure differential across the rolling diaphragm 5. A. valve 37, the opening and closing of which is actuated by the movement of the diaphragm 12, is arranged in the duct 35. If by some cause or other, an insufficient amount of liquid is present in the space 6, which may occur in starting the device, the diaphragm 12 will assume such a position that the valve 37 is opened so that medium from the container 36 enters the space below the rolling diaphragm through the duct 35. The rolling diaphragm is further supported-by said medium. if no or substantially no pressure variations 'occur in the space 4, the medium in the container 36 may be a -gas.,1n the alternative case it is desirable to use liquid as a-medium. In this manner it is reached again, that at those instants at which the pumping ring 9 supplies an insufficient amount of liquid, or has not yet been capable of supplying a sufficient amount of liquid, supporting medium is obtained from another source. in the device shown in FIG. 4, the supporting medium is temporarily obtained from an additional high-pressure container 36. v

The device shown in FIG. 5 again comprises a rolling diaphragm 5 which faces the gas space 4 with its convex side.

This means that for a good support of the rolling diaphragm the liquid pressure in the space 6 must always be higher by a given amount than the gas pressure. This is reached by arranging the spring 31 overthe piston 30 so that pressure forces which are directed towards the liquid; act upon said piston. The further construction and operation of said device correspond to that of the device shown in FIG. 3.

In those cases in which a liquid or a gas is applied during the starting period below the rolling diaphragm from an additional container, these additional containers must, of course be provided with the required liquid and gas, respectively, during operation. This may be effected by means of a pumpor a compressor which is coupled to the shaft of the device.

FIG. 6 shows a device which in outline corresponds to those municates with a liquid container 41 through a duct 40. in the duct 40 a pump 42 is arranged'which transports liquid from the space 41 to the liquid space 6 andcan supply more liquid per unit time than the pumping ring-.9. The control of this supply of liquid to the space 6 can be effected in two manners. Firstly, the pump 42 may be driven intermittently in that sense, that, dependent upon the position of the diaphragm 12, the control 43 actuates or deactuatesthe driving. Secondly, the pump 42 may run continuously, a controllable valve 44 being arranged in the duct 40 and, in addition, a circulating duct 45 being arranged across the pump 42. In this device the controllable valve is operated by the control device 43 in accordance with the position of the diaphragm 12. If insufficient if, by some cause or other an insufficient amount of liquid is present in the spaces 6 and 13, the gas pressure in the space 15 will ensure that the piston 30 is forced downwards. As a result of this, first of all the valve 34 will be forced on its seating and furthermore liquid will be forced from the space 13 to the space 6 so that the rolling diaphragm remains readily supin the correct manner, the pressure of the liquid in the space 6 will always have to be higher by a certain amount than the pressure of the gas in the space 4. This is reached by arranging the spring 22 in the control device 10 over the diaphragm 12.

When the pressure of the liquid in the space 6 becomes too liquid is present in the space6, the deficiency is very rapidly replenished by the pump 42.

FIG. 7 shows diagrammatically and not to scale a hot-gas reciprocating engine of the displacer type. This engine comprises a cylinder 70 in which a piston 72 and a displacer 73 reciprocate. The piston 72 and the displacer 73 are connected by a piston rod 74 and a displacer rod 75, respectively, to a gear not shown which moves said members with the desired mutual phase. The piston 72 bounds with its upper side a compression space 76 while the displacer 73 bounds with its upper side an expansion space 77. The compression space 76 and the expansion space 77 communicate with one another through a cooler 78, a regenerator 79, and a heater80. Below the piston 72 there is a buffer space 81 of such a volume that the volume variations in. this space produced by the movement of the piston 72 result in substantially no pressure variations. The seal between the piston 72 and the cylinder 70 is formed by a rolling diaphragm 82 and the seal between the piston 72 and the displacer rod 75 is formed by a rolling diaphragm 83. Below the rolling diaphragm 82 there is a liquid space 84 to which liquid is supplied through a pumping ring 85. Below the rolling diaphragm 83 there is a liquid space 86 to which liquid is supplied by a pumping ring 87. The space 84 communicates, through a duct 90 with a control device of the same construction as shown in FIG. 1. This control device further communicates with the compression space 76 through a duct 91. The operation of this control is entirely identical to that shown in FIG. 1 and will not be further described.

The liquid space 86 communicates through a duct 92 which comprises a spring-loaded check valve 93 with the compression space 76. Furthermore, an aperture 94 is provided in the wall of the liquid space 86 through which aperture liquid can be conducted away from the space 86 if the pressure differential across the rolling diaphragm 83 would become too small. If on.the contrary the pressure differential across the rolling diaphragm 83 becomes too large, the check valve 93 will be opened so that gas from the compression space 76 can enter the space 86 below the rolling diaphragm.

When the hot-gas reciprocating engine has been inoperative for some time, the liquid from the spaces 84 and 86 will have leaked away. in starting the device the valve 96 in the duct 97 will be opened, so that the working space is made to communicate with a space in which the same average pressure prevails as in the working space. Then the desired operating pressure can be adjusted in the working space; the control valve 93 and the valve 95 are opened because no liquid is present in the spaces 84 and 86, so that also gaseous medium is admitted to these spaces; thus during starting, at least at the beginning thereof, the rolling diaphragms 82 and 83 are supported by a gas. However, this may be'done readily in this case since, because the working space communicates with another space, no pressure variations will occur during the starting period so that the gas support for the rolling diaphragms will be sufficient.

FIG. 8 shows another embodiment of the hot-gas reciprocating engine shown in FIG. 7. In this engine the seal between the piston 72 and the cylinder 70 is formed by the. piston rings 100. The seal between the piston 72 and the displacer rod 75 is formed by piston rings 101. The seal between the piston rod 74 and the wall of the buffer space 81 is formed by a rolling diaphragm 102, while the seal between the displacer rod 75 and the piston rod 74 is formed by a rolling diaphragm 103. Below the rolling diaphragm 102 there is a liquid space 104 to which liquid can be supplied by a pumping ring 105. Below the rolling diaphragm 103 there is a liquid space 106 to which liquid can be supplied by a pumping ring 107. Above the rolling diaphragm 102 there is the buffer space 81, while above the rolling diaphragm 103 the space 108 is present which, through an aperture 109, is in open communication with the buffer space 81 so that, both above the rolling diaphragm 102 and above the rolling diaphragm 103, no pressure variations will occur. The liquid spaces 104 and 106 are in open communication with one another through an aperture 110, while these two spaces which are in open communication with one another communicate with the buffer space 81 through a duct 111 comprising a spring-loaded check valve 112. The dissipation of liquid from the two liquid spaces 104 and 106 may be effected through an aperture 113 which is opened and closed, respectively, by the rolling diaphragm 102, After the above it will be clear that this hot-gas reciprocating engine also can be started immediately with the desired operating pressure, independent of the fact whether a sufficient amount of liquid is present below the two rolling diaphragms. If an insufficient amount of liquid is present from the space 81 can enter the liquid space.

It will be clear that, besides during starting, the meaure according to the invention is also useful in the case of disturbances of liquid supply to the liquid spaces occurring during the operation. In fact, in that case the deficiency of liquid will also be replenished by medium, gas or liquid, which is obtained from another source.

Although in the drawings the liquid supply means are always represented by oil-pumping piston rings, it will be clear that instead of these pumping rings also a normal oil pump coupled to the shaft of the device may be used.

Iclaim:

1. in an apparatus having:

a. a housing including a cylindrical bore;

b. an element disposed coaxially within the bore and axially movable therein, and means operable with said element for moving same;

c. a rolling diaphragm seal secured between the element and the bore for defining and separating principal gas and liquid spaces for containing gas and liquid fluid mediums, the seal having a concave side and an opposite convex side, one of the sides facing the principal gas space and the other side facing the principal liquid space; and

d. pumping means responsive to movement of said element for supplying liquid to said principal liquid space upon said movement during normal operation of the apparatus for supporting the side of the diaphragm adjacent the liquid, the quantity of the liquid and the corresponding pressure across the diaphragm being variable on occasion between excessive and insufficient, a control device comprising (i) a control housing, (ii) first and second duct means respectively interconnecting the control housing with said principal gas and liquid spaces for communicating pressure variations and fluid mediums therethrough, and (iii) means in the control housing (A) responsive to said excessive pressure and operative to flow liquid out of the liquid space; and (B) responsive to said insufficient pressure and operative to flow gas from said gas space to said liquid space. 2. An apparatus as claimed in claim 1 wherein said first and second duct means have a low resistance to flow of said fluid medium.

3. An apparatus as defined in claim 1 wherein the pressure of the fluid medium supplied to the principal liquid space has higher pressure than the pressure prevailing in the principal liquid space, and the apparatus further comprises in association with said duct means a controllable valve which is opened to permit the flow of said medium when said pressure differential across said rolling diaphragm seal becomes excessive relative to said nominal pressure differential.

4. Apparatus as defined in claim 1 wherein the concave side of the diaphragm faces the principal gas space.

5. Apparatus as defined in claim 4 wherein said control device forms a control chamber having separate control spaces for containing gas and liquid respectively, said first duct interconnecting said principal gas and control gas spaces, said second duct interconnecting said principal liquid and control liquid spaces, the device further comprises a third duct interconnecting the first and second ducts, and a valve means operable with said first and third ducts for permitting said flow of gas therethrough from the principal gas space to the principal liquid space. 

